Lacustrine turbidites produced by surficial slope sediment remobilization: A mechanism for continuous and sensitive turbidite paleoseismic records

Turbidite records along ocean margins and in lake basins are increasingly used as paleoseismic proxies. However, the slope remobilization processes that formed the earthquake-related turbidity currents are poorly understood and, therefore, it is difficult to assess the consistency of turbidite paleoseismic records. We analyzed the sedimentary imprint of the giant AD 1960 (Mw 9.5) Chile earthquake in four South-Central Chilean lakes. Mass-transport deposits and turbidites were compared by interpreting reflection-seismic profiles, sedimentary facies, volume balances and proxies for turbidite composition (i.e. grain size, radionuclides, sediment color, organic matter). Sediment lightness was used as a proxy for organic matter content. Comparison of lightness of turbidites and the underlying sediments shows that earthquake-triggered turbidites in the four lakes result from remobilization of a thin veneer (on average about 5 cm) of slope sediments and do not result from disintegration of subaquatic landslides. Statistical analysis of 34 turbidites in a new 5200 year sedimentary record does not show any correlation between inter-event time and turbidite thickness (and thus volume). This means that inter-event slope recharging is not a dominant factor governing turbidite recurrence and that initiation of turbidity currents by remobilization of surficial sediments is different to slope failure related to subaquatic landslides. We conclude that surficial slope sediment remobilization forms a valid mechanism that explains why turbidite paleoseismic records can be of excellent continuity and high sensitivity. Moreover, based on correlations between seismic intensity, turbidite volume and stratigraphic remobilization depth, we propose that surficial slope sediment remobilization allows for turbidite records containing information about paleo-earthquake intensity.